In the valve arts a variety of valves are known which are intended to provide fluid flow at varying fluid pressures. The naturally occurring relationship of fluid flow to pressure (flow is proportional to the square root of pressure) can be regarded as unacceptable in a variety of technical contexts. For example, in medical ventilators used to assist patient breathing, and in similar devices used to supply breathing gas to a person for other reasons, it has been common practice to incorporate a fixed leak (i.e. a leak path of fixed cross-sectional area), especially in single tube circuits, to flush away the gas exhaled by the patient into the gas supply stream before the next patient inhalation begins. It is also known to utilize some such systems with an exhalation pressure lower than the inhalation pressure. In this case, the size of the fixed leak needed to flush exhaled gas from the supply tube under the low pressure conditions of exhalation may be so large that the flow rate through the fixed leak at higher pressure, as during inhalation, would result in enormous waste of supply gas during inhalation, For example, in one known gas supply system, a leak flow rate of about 20 liters per minute (L/min.) may be sufficient to flush exhalation gas from the supply conduit prior to the subsequent patient inhalation. A fixed orifice which will support a 20 L/min. leak at the system exhalation pressure of 2 cm H.sub.2 O will pass approximately 77.5 L/min. at 30 cm H.sub.2 O. Such a high leak rate during inhalation is not only wasteful of medical resources, but is also not required for any therapeutic reason.
It would be preferable that the leak which is provided to flush exhaled gas from such a system not increase appreciably with increases in pressure. Indeed, it might well be preferable that the leak flow rate during inhalation, if that is the higher pressure part of the system operating cycle, should be less than the leak flow rate during exhalation since there is no need to flush exhaled gas during inhalation. A varying system leak flow rate can also complicate operation of such known respiratory gas supply systems as those which are controlled in part by monitoring of the average system leak.
Among the prior art of valves purporting to regulate flow by means of pressure actuated regulators are those disclosed in U.S. Pat. Nos. 3,467,136, 3,474,831, 3,592,237, 3,948,289 and 3,951,379. Other flow regulating valves are disclosed in U.S. Pat. Nos. 3,429,342, 3,473,571, 3,770,104, 4,182,371, 4,234,013, 4,280,527, 4,351,510 and 4,354,516.